greybus: audio: fix to resolve multiple audio module playback issue

Cleanup APBridge sequence only in case of last module plugged-out.
For other modules, unregister cportid is sufficient.

Signed-off-by: Vaibhav Agarwal <vaibhav.agarwal@linaro.org>
Reviewed-by: Mark Greer <mark.greer@animalcreek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>